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Multiwalled carbon nanotubes are tubular carbon structures built from several concentric graphene shells, with an outer diameter of roughly 5 to 50 nanometres and lengths that can reach millimetres. They are produced mainly by chemical vapour deposition of a hydrocarbon over iron, cobalt or nickel particles, and whether a tube grows from its base or from its tip depends on how the catalyst interacts with the support, which also decides whether an aligned array or a tangled powder is obtained ^[1], and the carbon source and the growth temperature set that outcome ^[2]. The shells are held together only by weak interlayer forces, so the inner tubes can be pulled out and the outer ones thinned by oxidation, and the as-grown material usually contains catalyst residues and wall defects that have to be removed or at least quantified before use ^[3]. In a composite these tubes are added to raise stiffness, strength and conductivity, but the gain is limited by the interface: load and current have to cross a weak tube-matrix boundary, so surface treatment and dispersion control the result as much as the filler content does ^[4], and in a polymer it is the adhesion at that boundary that transfers the load ^[5].